Polyester has been used for various applications because it is superior in mechanical characteristics, thermal characteristics, chemical resistance, electrical characteristics, and moldability.
However, since polyester deteriorates with respect to mechanical characteristics due to hydrolysis, various studies have been made to control hydrolysis in the case of using it for a long term or in the case of using it under moist conditions. In particular, since films for solar batteries are required to have a lifetime of 20 years or more for outdoor use, they are required to be high in hydrolysis resistance and flame retardancy.
For example, a process for producing a polyester containing a phosphoric acid salt of an alkali metal or an alkaline earth metal has been disclosed in JP 2001-114881 A as a means to enhance hydrolysis resistance.
JP 2007-277548 A discloses a process for producing a polyester containing an inorganic phosphoric acid salt. In the Examples, it is used together with phosphoric acid.
JP 2008-7750 A discloses a polyethylene terephthalate containing a buffer phosphorus compound. In the Examples, it is used together with a phosphorus compound.
However, the use of only a metal phosphate as in the production process of JP '881 can control the initial amount of COOH terminal groups, but it is difficult to control increase in the amount of COOH terminal groups due to hydrolysis. Therefore, sufficient hydrolysis resistance cannot be obtained in applications with which long-term durability is required, such as a solar battery application.
In the production process of JP '548, since the proportion and the application amounts of phosphoric acid and an inorganic phosphoric acid salt are inappropriate, the inorganic phosphoric acid salt is heterogenized easily and mechanical characteristics of a film deteriorate due to a foreign matter. In addition, the film is superior in short-term hydrolysis resistance, but it is insufficient in long-term hydrolysis resistance, which is required for solar battery applications or the like.
In the production process of JP '750 since the type, the proportion, the application amount, and so on of a phosphorus compound are optimized insufficiently, products are insufficient in hydrolysis resistance and mechanical characteristics for solar battery applications.
It could therefore be helpful to provide a polyester composition suitable for films superior in hydrolysis resistance and in mechanical characteristics.